Force distribution due to impact of bidisperse granular avalanches against a retention barrier

Landslides, rock, debris, and snow avalanches are frequent disasters characterized by their destructive power. Retention structures are therefore essential for mitigating their effects, particularly near urban areas. Although widely studied, the role of grain-size polydispersity remains poorly addressed, even for the simplified case of bidisperse mixtures. In this work, we experimentally investigate the force histories generated by dense, dry, bidisperse granular flows impacting a rigid barrier at different relative concentrations C of the finer fraction. Experiments were conducted in an inclined flume, where a granular mass collided with a downstream rigid wall. A central strip of load cells measured the local force simultaneously under varying conditions. Time series of the force distribution, peak values, and the net force on the barrier were analyzed, showing a strong dependence on particle concentration, with maximum values often occurring near C=20%. This behavior is interpreted as the outcome of size segregation and its inherent asymmetry, which can be incorporated into dynamic force predictions using a Bagnold velocity scaling. Deposit profiles were also characterized, showing significant morphological variability with respect to C. Finally, we propose empirical relations linking maximum loads to deposit properties, providing scaling laws that may be applied to the design and assessment of real retention structures.